The invention is based on a circuit arrangement for operating an electric motor in a direct voltage network, in particular for operating a permanent-magnet-excited DC motor in the direct voltage network of a motor vehicle, of the kind that has been disclosed by German Patent Disclosure DE 101 17 252 A. This reference shows an electronically commutated two-phase electric motor, with a rotor excited by permanent magnets and with two windings in the stator that are supplied with current in alternation by an electronic controller via power switches, in which to regulate the electric motor, individual current supply periods per unit of time are suppressed by the electronic controller. The motor rpm is predetermined by the electronic controller from the comparison between an actual rpm and a set-point rpm, but the reference provides no information about the determination of the actual rpm. Nevertheless, the use of Hall transducers for this purpose is well known.
In addition, electric motors with electronic commutation are also known for use in motor vehicles; they have a rotor equipped with permanent magnets and a stator that carries the windings. In the motor vehicle, the areas in which such motors can be used are particularly in the field of ventilation, pumps, and adjusting drive mechanisms. The supplied current and the magnitude of the load moment determine the rpm; the rotary motion results from the supply of current to the stator windings, controlled on the basis of the rotor position, from a direct voltage network. The electronic controller is as a rule embodied by a microcontroller (μC) or a digital signal processor (DSP), and the commutation is controlled by a rotary position transducer whose exact positioning is essential if the commutation time of the motor is to be maintained in operation.